1. Field of the Invention
This invention relates to a method and apparatus for removal of outer polymeric coatings and inner tin coatings from metal surfaces. In particular, this invention discloses a method and apparatus for electrochemical removal of both an outer polymeric coating and an inner tin coating from a metal substrate. The invention finds utility in recycling post-consumer tin cans without generation of environmentally hazardous waste.
2. Brief Description of the Prior Art
Three million gross tons of tin-coated steel cans are produced in the United States per year, primarily as packaging for the food and beverage industry. Continued growth of the market share for tin cans in these industries is expected, as tin cans are convenient, safe, and cost effective. However, continued growth in these markets also leads to concerns regarding disposal and recycling of post-consumer tin cans.
Currently, approximately fifteen percent (four hundred fifty thousand gross tons) of all tin-coated steel cans are recycled. This percentage is expected to increase dramatically over the next five to ten years, due to concerns over landfill space and the necessity to treat incinerator ash containing cans as special waste under present environmental regulations. Unfortunately, these recycled cans may have poor markets due to their low value as raw materials.
This low value arises from the nature of tin-coated steel cans manufactured for the consumer market. Most of these tin-coated steel cans (hereinafter "tin cans") have a thin, electroplated coating of tin on both sides of the can for corrosion resistance. A few types of cans have a tin-free coating of chromium instead of tin, and others may contain small amounts of other metals. Hazardous metals such as lead are not currently used for food and beverage packaging. Lead is however used in cans for non-food packaging. Most tin cans further have a polymer resin coating or enamel (hereinafter "lacquer") on the inside of the tin can. Four major categories of lacquer coatings are in use today in the United States, including oleoresins, epoxy phenolics, vinyls, and polyesters. Approximately eighty-five percent of all post-consumer tin can wastes have lacquer coatings, sixty percent of these being epoxy phenolic-type coatings.
The steel used in the manufacture of tin cans is very high quality "low residual" steel, meaning that it contains minimal impurities. ("Low residual" steel as used herein refers to steel having less than 0.04% Ni, 0.06% Cr, 0.05% Mo, 0.02% As, 0.02% Cu, 0.15% P, 0.12% C, and 0.02% other impurities by weight). This low level of impurities allows the steel to be rolled into thin sheets without tearing. Tin is a highly detrimental contaminant in steel because it causes a condition called "hot shortness", which results in embrittlement and tearing during the steel-working operations. Thus the tin coating on tin cans must be removed in order to render them recyclable as a desirable source of steel, preferably to a level below about 0.06% by weight.
A detinning industry for removal of tin from other metals currently exists in the United States, but it is directed to the detinning of trimmings and scrap from new tin plate manufacture and product manufacture. These sources represent a very clear, consistent, uniform, and homogenous feed material. Accordingly, the emphasis in these plants has been the production of an extremely pure tin, rather than clean steel. The large centralized plants that currently exist are based on economy of scale, and have little tolerance for variation in feed material. Such industry is also unsuited for the recycling of post-consumer waste tin cans, due to high transportation costs, difficulties in effecting mechanical size reduction, and difficulties in removing lacquer coatings and other contaminants arising from labels and food residuals.
Transportation costs are high for recycling post-consumer tin cans in these facilities because of the facilities' central location. Existing methods for preparation and sizing of the tin cans tends to ball the metal up into nuggets that prevent even exposure of the tin surfaces for removal. Tin removal is also complicated by the lack of safe, economical methods for removal of the lacquer coatings. Thus the tin cans that are collected for recycling today are often begrudgingly accepted as-is (with the tin coating) in small quantities and for low prices, or surreptitiously hidden and blended into better grades of steel, or exported as one-time distress shipments to foreign mills.
As more recycling programs are instituted, and as consumer participation increases, it is unlikely that the steel industry will be able to absorb the increasing quantities of post- consumer waste tin cans as currently provided. There is even concern in the industry that increasing levels of these cans in the scrap cycle will lead to a dangerous increase in the tin levels of scrap steel, resulting in a temporary curtailment of all steel recycling in favor of virgin pig iron.
Prior detinning methodology makes use of chemical processes, wherein the tin is chemically stripped from the steel. Such chemical process technologies most commonly use a solution of sodium hydroxide to remove tin from tin plate, and then reclaim the tin from solution by electrolysis. The lacquer compositions commonly used for tin cans resists such treatment, reducing the effectiveness of the process. These processes also result in stripping steel from the cans, which is unrecoverable, and which contaminates the baths with iron precipitates. The chemical baths used for detinning are often highly contaminated, and present waste disposal difficulties in and of themselves.
Chemical detinning methods are disclosed in U.S. Pat. No. 4,164,542 to Deren, which uses a solution of highly concentrated sodium hydroxide and sodium nitrate or nitrite heated to 226.degree. F. to 236.degree. F.; and in U.S. Pat. No. 3,660,077 to Ruf, which uses a solution of soda lye and sodium chlorite. Other chemical methods included those disclosed in DE Patent No. 1546126 assigned to Goldschmidt AG (heating in a caustic solution containing a diethyleneglycolmonoalkylether); GB Patent No. 1594253 assigned to Metal Box Co. Ltd. (heating in a caustic solution containing an anthraquinone and an iron complexing agent); GB Patent No. 1594252 assigned to Metal Box Co. Ltd. (heating in a caustic solution containing an anthraquinone and an oxygenated alcohol as a delacquering agent); and U.S. Pat. No. 5,035,749 to Haruta et al. (using a solution of an aromatic nitro compound and then an inorganic acid solution to remove tin from copper).
Other methods for detinning include vacuum deposition (U.S. Pat. No. 4,227,922 to Laws et al.), and heating copper scrap in the presence of anhydrous hydrogen chloride gas (U.S. Pat. No. 5,035,749), and electrochemical removal. For example, electrochemical removal of a tin layer from copper is described in U.S. Pat. No. 4,264,419 to Pryor. This method requires the presence of a stannous ion complexing agent in order to maintain the tin concentration below about 10.sup.-4 gm-ions/L. U.S. Pat. No. 4,056,450 to McCauslan et al. also discloses an electrochemically-based process for detinning clean tin plate scrap, wherein the scrap is continuously advanced through an electrically anodic rotating perforated drum partially immersed in a caustic solution. The removed tin is plated upon cathodes disposed within the tank.
While suitable for removal of tin from relatively clean surfaces, the above-described methods are not suitable for detinning tin cans, because they do not efficiently remove the lacquer layer. The scrap described in McCauslan et al. has no lacquer coating, no labels, and no food residues to interfere with complete tin removal, and the patent notes that certain samples of heavily lacquered scrap yield treated scrap with unacceptably high levels of remaining tin (col. 5, lines 22-25). Accordingly, the process described in McCauslan et al. is unsuitable for treatment of post-consumer waste tin cans.
A few patents have been directed to the problem of lacquer or other coating removal. U.S. Pat. No. 4,474,655 to Giezen discloses a method of detinning painted tin plate waste, wherein the waste is first compressed into bundles, the bundles are immersed in a caustic bath for about sixteen hours in order to soften the paint, and the tin is then removed electrolytically without actual removal of the paint. However, this method requires long cycles, on the order of twenty-four hours per cycle. Other methods available for removal of polymeric layers from metal substrates also have disadvantages.
For example, chemical removal by oxidation in hot, alkaline solution requires high temperatures (greater than 200.degree. F.) that are difficult and costly to maintain in an industrial setting and which results in premature or unnecessary tin, lead or other metal removal. Highly caustic solutions present further operational difficulties. Incineration and dissolution in organic solvents present regulatory difficulties, requiring either air permitting or disposal of hazardous wastes. Incineration further results in migration of tin and iron to create a stable, unremovable tin-iron alloy throughout the tin layer. Cryogenic processing, which requires cooling to -196.degree. F., followed by mechanical shattering, is not cost-effective. Sonication to remove the outer polymeric coating from tin cans after freezing is described in Belgium Patent No. 852010 assigned to British Steel Corp.
Accordingly, there remains a need for the development of methods and apparatus suitable for smaller, more localized, yet still economical methods for recycling post-consumer tin cans. Such methods must further be environmentally benign, generating a minimum of waste requiring subsequent, specialized disposal methods.